she is a non character basically she is sayaka's girlfriend who was not in the high school part of the manga or anime
She is most likely a character who showed up at the end of the 2nd Sayaka spin-off novel that wasn't named at the time. We haven't seen a picture of her, but she'll probably feature prominently in the third book.
Oh...okey.
um do you by chance have a link you could post to the Sayaka spin off books ?
There isn't an official translation yet, but This blog has synopses and last I checked at least partial fan translations.
https://greatpassage.wordpress.com/2018/11/16/yagate-kimi-ni-naru-saeki-sayaka-ni-tsuite/
Um...who's haru chan...i cant memorize some of the character